The magnetic recording industry has increased the performance and capacity of hard disk drives to meet the demands of the computer industry for more and better storage. Applications such as multimedia, real-time audio and video, graphical user interfaces and increasing program sizes are driving this increase. Hard disk areal density storage capacity historically increased at an average yearly growth rate of approximately 25 percent. Sustaining this growth in capacity has required progressive advances in many technologies used to provide a hard disk drive.
Historically, read-write head technology was based on the inductive voltage produced when a permanently magnetized area on a rotating disk moved past a head employing a wire-wrapped magnetic core. Increasing areal density requirements drove a steady progression of inductive recording head advances, which led to advanced thin-film inductive read-write heads.
The inductive head is frequently expected to alternatively perform the conflicting tasks of writing data onto the disk and reading previously-written data. In other implementations, the write and read functions are separated into two physically distinct heads. This allows using an inductive head that is optimized for writing data and a magnetoresistive head structure that is optimized for reading data. In such an apparatus, the magnetoresistive read head includes of a read element that is sandwiched between two highly-permeable magnetic shields. The shields assist in focusing the magnetic energy from the disk and rejecting stray fields. The magnetoresistive read element is made from a ferromagnetic alloy whose resistance changes as a function of an applied magnetic field. In a hard disk drive, this magnetic field is derived from the magnetized regions placed on the rotating disk by the write head and is used to modulate the resistivity of the magnetoresistive read element during a read operation.
A schematic diagram of an example prior art implementation of a voltage biasing circuit for biasing a magnetoresistive read element RMR is illustrated in FIG. 1. The example implementation provides a voltage defined bias. In other words, the circuit provides a set voltage and sources a current based on the resistance of the magnetoresistive read element RMR. The circuit outputs a differential voltage Vdiff that corresponds to the high frequency variation of the resistivity of the magnetoresistive read head RMR caused by the magnetized regions of a hard disk.
The example circuit of FIG. 1 includes a voltage source Vsource, an impedance Rp, a current source Idac, a impedance Rc1, a impedance Rc2, a transistor T1, a transistor T2, a current source Itail1, a current source Itail2, a magnetoresistive read head RMR, a transistor M1, a transistor M2, a transistor M3, a transistor M4, a capacitor C1, and an opamp OP1.
The voltage source Vsource, the impedance Rp, and the current source Idac provide a set bias voltage to the transistor T1 and the transistor T2. The bias voltage allows current to flow from a positive supply voltage Vdd through the impedance Rc1 and the impedance Rc2 and through the transistor T1 and the transistor T2 respectively. When, the transistor M1 and the transistor M3 are not biased, some of the current flowing through the impedance Rc1 flows through the magnetoresistive read head RMR. The current source Itail1 is set to source the amount of current flowing through the impedance Rc1 minus the amount of current flowing through the magnetoresistive read head RMR. The current source Itail2 is set to source the amount of current flowing through the impedance Rc2 plus the amount of current flowing through the magnetoresistive read head RMR.
When the transistor M1 and the transistor M3 are not biased on, but the impedance of the magnetoresistive read head RMR changes (e.g., due to subjecting the magnetoresistive read head RMR to a magnetic field), the amount of current flowing through the magnetoresistive read head RMR changes. The change in current causes the current flowing through the impedance Rc1 to increase by the amount of the change (e.g., an increase if the current flowing through the magnetoresistive read head increases and a decrease if the current flowing through the magnetoresistive read head decreases). The change in current develops a voltage potential between a first node between the impedance Rc1 and the transistor T1 and a second node between the impedance Rc2 and the transistor T2 (voltage differential Vdiff).
The voltage differential Vdiff between the first node and the second node is connected to the opamp OP1. The opamp OP1 outputs a voltage proportional to the difference between the inputs. The voltage from the opamp OP1 causes the transistor M3 to be biased on, which sinks current from the magnetoresistive read head RMR. The voltage from the opamp OP1 also biases the transistor M4, which causes a gate of the transistor M2 and a gate of the transistor M1 to be tied to ground. The transistor M1 is biased into operation, which allows current to flow through the transistor M1 and into the magnetoresistive read head RMR. Accordingly, the transistors M1 to M3 respectively source and sink current through the magnetoresistive read head RMR such that the current flowing through impedance Rc1 and impedance Rc2 is unaffected by a variation in the resistance of the magnetoresistive read head RMR. The capacitor C1 sinks high frequency signals from the opamp OP1 to ground. Therefore, the opamp OP1 only controls for resistivity variation at low frequencies, which allows the high frequency variations (e.g., variation due to the difference between natural resistivities of magnetoresistive read heads) caused by the magnetoresistive read head RMR passing over magnetized regions of a hard disk to be detectable at the output Vdiff.